The 19-year-old was handed a full senior debut for the Whites against West Ham in the third round of the FA Cup on Sunday afternoon.
Greenwood was substituted at half-time in the 2-0 defeat at the London Stadium and has now revealed he picked up an issue in the capital though no further details were provided.
"Yesterday wasn’t meant to be, but it’s a privilege to make my first start for Leeds United," he wrote on social media.
"Thanks for the support, unbelievable as always. Gutted to have picked up a injury but I’ll be back stronger."
Marcelo Bielsa handed three of Thorp Arch's youngsters - Greenwood, Lewis Bate and Leo Hjelde - full debuts against the Hammers due to mounting injuries in West Yorkshire.
Leeds have been left severely short across the board and, particularly, up front due to problems for Patrick Bamford, Tyler Roberts, Joe Gelhardt and Rodrigo.
Kalvin Phillips and Liam Cooper are also longer absentees while Jamie Shackleton and Charlie Cresswell are battling for fitness alongside Pascal Struijk.
To add to the Whites woes, full-back Junior Firpo left the field of play against West Ham with an apparent head knock.
Diego Llorente is also suspended for the second visit to face David Moyes' men this weekend in Premier League action.